[jwilliams]

Stands for "to come" https://en.m.wikipedia.org/wiki/To_come_(publishing)

Theory is that TK was a less common letter combination than TC.

[jcheng]

In case anyone is curious, like I was:

  $ grep -i tc /usr/share/dict/words | wc -l
      1055
  $ grep -i tk /usr/share/dict/words | wc -l
        40

[random_comment]

(The agony of using 'TC' when you code in tcl, use tcpip, or are working with terminal control code)